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-   -   prob. with unsteady RANS using Finite Rate Chem (http://www.cfd-online.com/Forums/fluent/40826-prob-unsteady-rans-using-finite-rate-chem.html)

James May 4, 2006 05:35

prob. with unsteady RANS using Finite Rate Chem
 
Hi All,

I am having some problems with unsteady RANS using the finite rate chemistry combustion model. I am using the segregated solver and for steady RANS, i got a convergent solution. However, upon switching to unsteady RANS, i realised the temperature is increasing.

One suggestion i have seen from a past post is to use a time step that is close to the chemical time scale. How can i determine the chemical time scale?

I tried switching to the couple solver but that too is not helping. Using the implicit option, i get the message that i do not have enough disk space. Using the explicit option works but my residuals become very large and they do not seem to be dropping quickly enough.

Any suggestion(s) on how to remedy this problem will be greatly appreciated.

James


Kamarul Arifin May 4, 2006 10:11

Re: prob. with unsteady RANS using Finite Rate Che
 
estimation of chemical time scale = Length scale / velocity scale

James Willie May 4, 2006 12:42

Re: prob. with unsteady RANS using Finite Rate Che
 
Hi,

Thanks for the response. Which length scale are you referring to? Is it the integral length scale? Also which velocity scale?

Counting on your assistance.

James


Kamarul Arifin May 5, 2006 05:41

Re: prob. with unsteady RANS using Finite Rate Che
 
Length scale = smallest edge of mesh velocity scale = freestream velocity

Please noted that this is just a estimation of the chemical time scale.

reference : Tennekes text book

Hope this helps.


James May 16, 2006 07:29

text book
 
Hi,

Thanks so much for the hint.Can you give me the full title of the Tennekes book. I have this title with me but i am not sure whether it is the right one...Tennekes H and Lumley, J. L.' A first course in Turbulence' MIT press, Cambridge Massachusetts, 1972.

Please let me know if the title is the right one.

James



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